Sound of star's vibration reveals details of its core

Astronomers have made the most detailed observation yet of the subtle vibrations of a Sun-like star. The technique reveals details of the star's interior that cannot be studied any other way.

Stars pulsate slightly as churning gas in their outer layers creates low-frequency sound waves that rebound inside them. The frequency of each wave reveals the speed at which sound travels through a star's inner layers, shedding light on its density, temperature, composition and rotation.

These waves have been studied in just a dozen or so stars other than the Sun. Now, astronomers have used the Very Large Telescope in Chile and the Anglo-Australian Telescope in Australia to perform the most precise measurement yet of the star alpha Centauri B. It is one of a trio of stars known as Alpha Centauri, which are the Sun's closest neighbours, at a distance of just 4.3 light years.

Researchers led by Hans Kjeldsen of Aarhus University in Denmark took advantage of the fact that the two telescopes' are located on opposite sides of the Earth to provide nearly continuous observation of the star in the night sky. "That's a huge advantage, because gaps in the data introduce ambiguity," explains team member Tim Bedding at the University of Sydney.

Deepened voices

Once a minute for seven nights in a row, they watched how the star's light was stretched or shortened by the movement of the star's surface. From these observations they determined the frequency of the sound travelling through the star. The surface of the star was found to move by about a dozen metres in 4 minutes, generating 37 different types of sound wave.

By revealing the composition of a star's interior, such oscillations can help determine its age. As a star grows, its core fuses hydrogen into helium. And, as sound moves more slowly in helium than hydrogen, a star's "voice" effectively deepens as it ages, says the team.

Audio samples of the pulsations created by various star, including alpha Centauri B and the Sun (WAV audio format), can be downloaded from the researcher's website.

Journal reference: Astrophysical Journal (vol 635, p 1281)

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Though it looks like a single star, Alpha Centauri (the bright spot left of centre) is actually a triplet (Image: Claus Madsen/ESO)